The present invention relates generally to aircraft guidance systems and more particularly to an aircraft landing aid that employs a projected beam of light not only to define a glide slope along which a pilot may direct his aircraft in order to touch down at a specific area on a runaway but also to distinguish this projected light beam from other lights typically found in the area surrounding airports.
One type of aircraft landing aid known generally as a Pulse Coded Optical Landing Aid (PCOLA) is described in commonly owned U.S. Pat. No. 3,648,229, Pulse Coded Vehicle Guidance System, by Burrows, et al. In this system, a series of light choppers are used in conjunction with a source of light and a colored filter to present to the pilot a variety of visual signals to allow him to determine his position relative to the glide slope. If the pilot is within the glide slope, which typically is contained within a vertical angle of approximately 0.3.degree., the pilot will see a solid white light. If the aircraft is higher than the optimum glide slope but within about three degrees thereof the pilot will see a pulsating white light. Furthermore, the pulses of light will become longer and the intervals between pulses will be shorter as the pilot more closely approaches the glide slope, giving the pilot a visual indication of his angular deviation above the glide slope. Likewise, within an angular distance of about 3.degree. below the glide slope the pilot will see a pulsating red light where the duration of the interval between pulses will become greater as the pilot more closely approaches the glide slope. In this manner the pilot is presented with a continuous indication of his position in relation to the glide slope approaching the runway.
A problem arises in that the pilot is sometimes unable to pick out the Pulses Coded Optical Landing Aid from the clutter of lights, both blinking and steady, that may surround an airport. This is particularly true if the pilot is on course down the glide slope when he will see a steady white light. Some identification schemes have been suggested such as separated beacons but by their nature have been expensive, unattractive and add to the clutter of lights around an airport.